The primary objective of the proposed research is to measure the extent of genetically determined structural heterogeneity within electromorphs (electrophoretic allozymes) of enzymes in house mice, humans, fruit flies, and bacteria in an attempt to estimate total allelic diversity at structural gene loci in these organisms. Allelic substitutions that do not alter electrophoretic mobility of polypeptides ("hidden variation") will be detected and characterized primarily be measuring thermostability but also by assessing electrophoretic response to treatment with thiol reagents. The methodological procedure involves using electrophoresis to reveal amino acid substitutions (affecting electrostatic charge for the most part) between proteins, calibrating heat-denaturation methods by comparing electromorph pairs, and then screening electromorphs for heterogeneity among individuals at this level of resolution. If heat-denaturation techniques can be calibrated at or near the level of single amino acid substitutions, it will be possible to estimate total allelic diversity at loci. The research is designed to test recently developed theories regarding the relationship between effective population size and the ratio of electromorphs to alleles. The research is expected to yield results important in determing the bases for the maintenance of molecular polymorphism in natural populations of organisms. Much of the work deals with the bacterium Escherichia coli.